Get a splitter for your sound-output (one male 1/8" stereo to two female 1/8" stereo), and a short extension cable (1/8" stereo male to 1/8" stereo male). These are easy to find and cheap to buy at Radio shack or other such stores.
Plug the splitter into your sound OUT on your mobo. Plug your speakers into one side of the splitter, and the extension cable to the other side. Then route the other end of the extension back into your Line-In or Mic port.
Now all you need is some basic recording software. There are tons out there, a lot of them free. Your Soundstorm software package may have even included one. Just set the recording program to record from whatever jack you routed your sound to (Line-In or Mic) and hit the record button and your all set.
Id reccomend getting it set up and do some test runs to make sure everything works ok before your friends show.